contacts. ROCK increases the activity of the motor protein myosin II by two different mechanisms: Firstly, phosphorylation of the myosin light chain (MLC) increases the myosin II ATPase activity. Thus several bundled and active myosins, which are asynchronously active on several actin filaments, move actin filaments against each other, resulting in the net shortenting of actin fibres. Secondly, ROCK inactivates MLC phosphatase, leading to increased levels of phosphorylated MLC. Thus in both cases, ROCK activation by Rho induces the formation of actin stress fibers, actin filament bundles of opposing polarity, containing myosin II, tropomyosin, caldesmon and MLC-kinase, and consequently of focal contacts, which are immature integrin-based adhesion points with the extracellular substrate. 2. Other functions and targets RhoA-GTP stimulates the phospholipid phosphatase activity of PTEN (phosphatase and tensin homologue), a human tumor suppressor protein. This stimulation seems to depend on ROCK. In this way, PTEN is important to prevent uncontrolled cell division as is exhibited in cancer cells. ROCK plays an important role in cell cycle control, it seems to inhibit the premature separation of the two centrioles in G1, and is proposed to be required for contraction of the cleavage furrow, which is necessary for the completion of cytokinesis. ROCKs also seem to antagonize the insulin signaling pathway, resulting in a reduction of cell size and influence cell fate. ROCKS play a role in membrane blebbing, a morphological change seen in cells committed to apoptosis. The pro-apoptotic protease, caspase 3, activates ROCK kinase activity by cleaving the C-terminal PH domain. As a result, the autoinhibitory intramolecular fold of ROCK is abolished. ROCK also regulates MLC phosphorylation and actomyosin contractility, which regulate membrane blebbing. ROCKs contribute to neurite retraction by inducing growth cone collapse by activating actomyosin contractility. It is also possible that phosphorylation of collapsin response mediator protein-2 (CRMP2) by ROCK inhibits CRPM2 function of promoting axon outgrowth, resulting in growth cone collapse. ROCKs regulate cell-cell adhesion: Loss of ROCK activity seems to lead to loss of tight junction integrity in endothelial cells. In epithelial cells inhibition of ROCK seems to decrease tight junction integrity. Active ROCK in these cells seems to stimulate the disruption of E-Cadherin-mediated cell-cell contacts by activating actomyosin contractility. 3. Other ROCK targets NHE1 (a sodium hydrogen exchanger, involved in focal adhesions and actin organisation) intermediate filament proteins: Vimentin, GFAP (glial fibrillaric acidic protein), NF-L (neurofilament L protein) F-actin binding proteins: Adducin, EF-1&alpha (elongation factor,